1) Field of the Invention
The present invention relates generally to an apparatus and method for analyzing unwanted power frequencies that are generated by pulse width modulation for reducing color segregation in high intensity discharge lamps.
2) Description of Related Art
High intensity discharge lamps (HID) are becoming increasingly popular because of their many advantages, such as high efficacy and brightness. These HID lamps are driven by either a high frequency electronic ballast that is configured to generate driving current signals above the 20 kHz range or by a low frequency electronic ballast with driving current signals in the 100 Hz range.
However, a major obstacle to the use of high frequency electronic ballasts for HID lamps is the acoustic resonances/arc instabilities which can occur at high frequency operation. Acoustic resonances, at many instances, can cause flicker of the arc which is very annoying to humans. Furthermore, acoustic resonance can cause the discharge to extinguish, or even worse, stay permanently deflected against and damage the wall of the discharge lamp. Techniques for stabilizing and centering this arc have been developed. U.S. Pat. No. 5,134,345 teaches the detection of arc instabilities and reducing the power to the lamp to stabilize the discharge. In U.S. Pat. No. 5,306,987, an arc stabilization technique is illustrated in which the frequency of the drive signal is modulated. A similar method of controlling the arc in discharge lamps is illustrated in U.S. Pat. No. 5,198,727. With this method, the arc is centered by the xe2x80x9cacoustic perturbationsxe2x80x9d induced by the frequency modulated HF (high frequency) ripple superimposed on the unidirectional current. The acoustic perturbations compel the gas or vapor movement patterns to counter the gravity-induced convection. U.S. Pat. No. 5,684,367 discloses a method of controlling arc destabilization in HID lamps by amplitude modulation of a high frequency signal and pulsing the lamp, which can be used to change the color characteristics of the lamp.
Recently, a new class of high intensity discharge lamps has been developed that employ ceramic (polycrystalline alumina) envelopes. The discharge envelope in this class of lamps is cylindrical in shape, and the aspect ratio, i.e., the inner length divided by the inner diameter is close to one, or in some instances more than one. The lamps which have an aspect ratio that is significantly greater than one have the desirable property of higher efficacy, but they have the disadvantage of having different color properties in vertical and horizontal operation. In particular, in vertical operation color segregation occurs. The color segregation can be observed by projecting an image of the arc onto a screen, which shows that the bottom part of the arc appears pink, while the top part appears blue or green. This is caused by the absence of complete mixing of the metal additives in the discharge. In the upper part of the discharge there is excessive thallium emission and insufficient sodium emission. This phenomena leads to high color temperature and/or decreased efficacy.
Commonly owned U.S. Pat. No. 6,184,633 entitled Reduction of Vertical Segregation In a Discharge Lamp, incorporated herein by reference, teaches a method to eliminate or substantially reduce acoustic resonance and color segregation by providing a current signal frequency sweep within a sweep time, in combination with an amplitude modulated signal having a frequency referred to as second longitudinal mode frequency. The typical parameters for such operation are a current frequency sweep from 45 to 55 kHz within a sweep time of 10 milliseconds, a constant amplitude modulation frequency of 24.5 kHz and a modulation index of 0.24. For example, in order to excite the 2nd longitudinal mode for a 70 W lamp having a 4 mm internal diameter and an inner length of 19 mm, a high frequency sweep from 45 to 55 kHz is amplitude modulated at about 24 kHz. When this waveform is generated by function generators and a power amplifier at low levels of modulation, the resulting power spectrum has frequency components at 24 kHz, 90 kHz to 110 kHz and side bands at 66 kHz to 86 kHz and 114 kHz to 134 kHz. In this ideal power spectrum, there are no power frequency components above about 150 kHz. It is the power frequencies that are important for exciting acoustic resonances.
Another system for reducing color segregation in HID lamps employs a very high frequency ballast bridge (e.g. 250 kHz) having a full-bridge or a half bridge configuration that is controlled by a pulse width modulation (PWM) signal generator. Such a system is shown in commonly owned and copending U.S. patent application Ser. No. 09/684,196, filed Oct. 6, 2000, which is hereby incorporated by reference. In addition to the desired frequency components shown above, this method also produces additional swept power frequencies centered around 200 and 300 kHz and with side bands on both sides of these swept power frequencies. The exact frequencies of the additional swept power frequencies depend on the bridge frequency and are produced from the sum and difference frequency terms of the bridge frequency and high frequency sweep. These additional power frequencies can produce arc instabilities that are not present when the ideal power spectrum is used. Although the ballast bridge frequency can be changed in order to see if the arc instabilities diminish, there are too many frequencies produced by the bridge to identify the offending frequencies. Introducing color mixing also can change the properties of the discharge and therefore change the offending frequencies.
Therefore, what is needed is an independent method and apparatus to identify the offending power frequencies and determine their threshold power level for causing arc instabilities.
The present invention utilizes frequency sweeping and amplitude modulation or sequential excitation to determine the power frequencies that cause arc instabilities in a high intensity discharge lamp. When a lamp is operated with a sinusoidal waveform at a current frequency X, the power frequency is at 2X. It is the power frequencies that are important for exciting acoustic resonances. The power frequencies applied to the high intensity discharge lamp are determined by the frequency dependence of the product of the current and voltage waveforms at the high intensity discharge lamp.
In one aspect, the present invention is directed to a method for determining which frequencies applied to a high-intensity discharge lamp cause arc instabilities, comprising the steps of (a) providing a signal having frequencies within a predetermined range of frequencies, (b) amplifying the signal, (c) inputting the amplified signal into a high intensity discharge lamp so as to effect application of power frequencies to the lamp, (d) determining if the power frequencies cause arc instabilities in the high intensity discharge lamp, (e) determining a minimum power level of the power frequencies determined in step (d) that is required to cause arc instabilities in the lamp, (e) changing the frequencies of the current signal to other frequencies in the range, and (f) repeating steps (b)-(e).
In one embodiment, the aforementioned providing step (a) comprises the steps of providing a first signal having a predetermined fixed frequency, providing a second signal that is periodically swept over a sweep range from a first frequency to a second frequency during a sweep time period, and summing the first and second signals to produce a sum signal having frequencies that are the sum of the frequencies of the first and second signals. The amplified sum signal is inputted into the high intensity discharge lamp. As a result, power frequencies based upon the sum and difference frequencies of the first and second signals are applied to the high intensity lamp as well as power frequencies at twice the first signal and twice the second signal.
In another aspect, the present invention is directed to a method for determining which frequencies applied to a high-intensity discharge lamp cause arc instabilities, comprising the steps of (a) providing a first signal having a predetermined fixed frequency, (b) providing a second signal that is periodically swept over a sweep range from a first frequency to a second frequency during a sweep time period, (c) summing the first and second signals to produce a sum signal, (d) amplifying the sum signal, (e) inputting the amplified sum signal into a high intensity discharge lamp so as to effect application of power frequencies to the lamp wherein the power frequencies include the sum and difference of the first and second signals, (f) determining if the power frequencies cause arc instabilities in the high intensity discharge lamp, (g) varying the amplitude of the first signal in order to determine the minimum power levels of power frequencies determined in step (f) that are required to cause arc instabilities, and repeating steps (b)-(g) for each fixed frequency required to probe a range of power frequencies.
In a further aspect, the present invention is directed to an apparatus for determining which power frequencies applied to a high-intensity discharge lamp cause arc instabilities, comprising a signal generator that produces a signal that is swept through a plurality of frequencies during a sweep time period, an amplifier for amplifying the signal, means for inputting the amplified signal into a high intensity discharge lamp so as to effect application of a range of power frequencies to the lamp, and a signal processing device for determining (1) the power frequencies applied to the lamp that cause arc instability in the high intensity discharge lamp, and (2) the minimum power level of such power frequencies required to cause arc instabilities. In one embodiment, the signal generator apparatus further comprises a first signal generating device for generating a first signal having a fixed frequency, and a second signal generating device for generating a second signal that is periodically swept over a sweep from a first frequency to a second frequency during a sweep time period, and the apparatus further comprises a summing network for summing the first signal and the second signal to produce a sum signal having frequencies that are the sum of the frequencies of the first and second signals. The amplified sum signal is inputted into the high intensity discharge lamp thereby causing power frequencies based upon the sum and difference of the frequencies of the first and second signals as well as power frequencies at twice the first signal and twice the second signal to be applied to the high intensity lamp.